Crawler tractors normally comprise a motor connected to a transmission device by a driving shaft. A main clutch is arranged along the driving shaft for selectively connecting or disconnecting the transmission device and the motor. The transmission device rotates a right shaft and a left shaft, connected respectively to a right drive sprocket and a left drive sprocket, each of which is arranged for moving a corresponding track. A brake and a steering clutch are associated to each drive sprocket in order to enable the operator to steer in the corresponding direction. In particular, if the operator desires to steer to the right along a relatively wide steering arc, he simply has to disengage the steering clutch associated with the right track. If it is instead desired to steer to the right along a relatively narrow arc, in addition to disengaging the right steering clutch the operator must also act on the brake associated to the right track. Similar operations have to be performed on the left track if it is desired to steer the vehicle to the left.
In known tractors, the transmission device is controlled by means of a plurality of levers, which comprise at least a first lever for engaging or disengaging the main clutch, a second lever for steering the tractor to the right or to the left, and a third lever for changing gear. Other levers are also normally provided for performing additional operations such as controlling the parking brake, the accelerator or particular implements driven by the tractor.
The known tractors of the kind described above are quite complicated to be driven, due to the great number of manual controls that have to be activated by the operator, sometimes even simultaneously.
In order to overcome this drawback, attempts have been made to provide a single control lever by means of which the operator can both engage or disengage the main clutch, and steer the tractor to the left or to the right. However, such attempts have never led to a device which can be easily adopted on an industrial scale and is easy to be controlled by an operator.
An object of the invention is to improve devices for controlling the transmission of a vehicle, particularly a crawler vehicle.
Another object is to provide a device for controlling the transmission of a vehicle by engaging or disengaging a main clutch of the vehicle, which device enables the operator to understand step by step in which position the main clutch is and can therefore be easily operated.
A further object is to provide a device for controlling the transmission of a vehicle, which device has a simple and compact structure and can therefore be easily manufactured on an industrial scale.
According to the invention, there is provided a device for controlling the transmission of a crawler vehicle, comprising:                a lever element which is actuatable by an operator, the lever element being continuously displaceable between a plurality of positions;        control means operable by the lever element for engaging or disengaging a main clutch of the vehicle;characterized in that the device further comprises a cam member cooperating with a rocker arm assembly for applying to the lever element a controlled force, said controlled force depending on the position of the lever element.        
During operation of the device according to the invention, the controlled force applied on the lever element by the cam member cooperating with the rocker arm assembly provides the operator with precise information concerning the position of the lever element, and consequently of the main clutch of the vehicle. The operator may therefore understand step by step how the device for controlling the transmission of the vehicle is moving, which makes the device particularly easy to be operated.
By properly selecting the profile of the cam member, it is furthermore possible to ensure that the force applied to the lever element is adapted to the specific application for which the device according to the invention is intended.
Owing to the cam member and the rocker arm assembly, a device can be obtained having a particularly simple structure, the device being made of a reduced number of mechanical components. This makes it easier mounting and repairing the device, if necessary. The device can therefore be easily manufactured and adopted even on an industrial scale.
If desired, the device according to the invention can be coupled to a steering system, so that the same lever element can be used both for engaging or disengaging the main clutch, and for steering the vehicle to the left or to the right. This further improves the vehicle control.
In an embodiment, the cam member is delimited by a cam profile having a recess for stably receiving a cam follower of the rocker arm assembly, in a position of the lever element corresponding to a configuration in which the main clutch is engaged.
In an embodiment, the cam profile further has a valley for stably receiving the cam follower in a stable neutral position of the lever element.
Thus, it is possible to obtain a device exhibiting two stable positions. In the first stable position, the clutch is completely engaged and the vehicle advances, whereas in the second stable position the clutch is disengaged and the vehicle is stationary. Theoretically, a number of stable positions greater than two can be provided.